A. Field of the Invention
The present invention relates generally to sports boards, and more particularly to polyethylene based composite foam boards for gliding on snow or water having a stiffening element coupled to the core for mechanical strengthening of the board. The present invention relates to stiffening elements composed of substantially polymer foam that may be placed in selected positions to selectively increase rigidity to selected portions of a foam board. With controlled placement of stiffening element, the foam board can be adapted to have higher stiffness in a first portion and greater flexibility in a second portion of the board.
B. Description of the Prior Art
Polyethylene foam sports boards have recently become very popular, in particular in the application as snow sleds, bodyboards and other kinds of gliding boards or the like. However, one drawback in using a polyethylene foam sports board is that it does not have the desirable stiffness against the flex of the foam board caused by the weight of the rider and this impairs the maneuverability of the sports board. For example, in the application of snow sled, such deformation of the board will result in the foam sled submerging below the snow surface and thereby reduce the sliding speed and directional stability when carrying a rider sliding down a snow-covered slope. It is obviously more desirable to have a foam sled or a bodyboard having higher flexural strength.
Some prior arts apply a foam core of higher stiffness to overcome the problem. For example, polystyrene (PS) foam has higher flexural strength at a similar density than does polyethylene (PE) foam. Due to the difference in chemical composition between the foam core, the polyethylene foam skin and polyethylene slick film, manufacturing requires special bonding material and techniques to bond the polyethylene skin to the foam core. That makes the manufacturing difficult and costly. Some manufacturers try to produce a polyethylene foam board with higher flexural strength by increasing the thickness or density of foam core, or by laminating a thick polymer film outside the foam board. Yet this increases the weight of foam board degrading desirable performance in its applications as a snow sled or bodyboard. It would be desirable to provide a method to make a polyethylene foam sports board at a lower density and less weight but with sufficient stiffness to support a rider to perform the intended function on the sports board. Also desirable is to provide a method to make a foam sports board that has high flexural strength at relatively low cost.
Polystyrene foam core has been used to produce foam sports boards due to its lightweight and rigid properties. However, there are some drawbacks associated with a polystyrene foam core. For example, in the case of snow sled application, a polystyrene foam core does not provide the same degree of comfort in terms of cushioning and shock absorption properties when compared with a polyethylene foam core. In addition, the polystyrene foam is poor in elastic recovery under even minor impact force, and may form undesirable indentations such as in the front portion of a snow sled. It may be desirable to develop a foam core that has the structure stiffness advantages of a polystyrene foam core in the main body and the desirable resilient and shock absorption properties of polyethylene foam in the top and front portion of the board.
One typical method of strengthening bodyboard is to insert one or more cylindrical rods, know as stringers, into holes drilled parallel to the longitudinal axis of the board from the tail end toward the nose end. A stringer system would generally include a fiberglass or graphite rod that is centrally inserted and adhesively secured in the foam core material. The disadvantage of the current stringer systems used is that the stringer may separate from the foam core after frequent use, in extreme conditions of use, for example when the board is bent up harshly, the stringer rod can pop out of either the deck or the bottom of the bodyboard. It would be desirable to provide a method to insert stiffening element by heat laminating to the foam core so that a strong bond forms between stiffening element and the foam core.
Typically, in use, the rider of a bodyboard grips the front edge close to one corner of lead nose and front portion of the side edge of the bodyboard. The hand in the front edge plays a significant role in steering control and maneuvering the bodyboard. Successful completion of maneuvers requires the bodyboard to respond adequately to the rider's steering. Force applied to the bodyboard that only distorts the board does not help the rider in redirecting the board. Thus, a high degree of stiffness of the bodyboard is desirable. However, it may not be desirable to make the bodyboard very rigid entirely from the nose to the tail. For example, it may be desirable for the board to be more flexible at the portion between a transverse line about a quarter of the way from the nose and the lead nose. Such flexibility allows the rider to pull up the nose of board and keep the nose and lead corners from dropping under the water's surface in a dynamic situation where the nose is being forced downwardly. However, in the forward quarter of the board, it is generally considered desirable for the board to be very stiff along a transverse line so that the rider's steering inputs on one side of the board will effectively be transmitted to the opposite side of the board and redirect the opposite side. It is therefore more desirable to provide an improved foam core with stiffening element adapted to increase the stiffness of a selected portion of the board. It is also desirable to provide an improved foam core with stiffening element adapted to provide the resistance to flex along a transverse axis and yet allow adequate flexibility along a longitudinal axis of the foam core in the forward quarter of the board.
The necessary condition to provide stiffening to a composite foam core is that the flexural strength of the coupled stiffening element has to be substantially higher than that of the low density base foam core. It is apparent that when applying polystyrene foam plate as the stiffening element to a polyolefin foam base core, the polystyrene foam plate contributes higher flexural strength and a stiffening effect. In the case of monolayer of high density polyolefin foam sheet, the stiffening effect is based on the fact that the compressive strength of plastic foam such as polyethylene foam can vary with direction relative to the direction of extrusion, the maximum strength being along the direction of extrusion. In the case of a sandwich structure of polyolefin foam/polyolefin sheet/polyolefin foam, the stiffening effect is mostly derived from the higher flexural strength of the solid polyolefin sheet.
In the process of foam sheet extrusion, the cells are stretched in the direction of the extrusion. The thin plastic film of each cell wall is oriented and strength hardened by the shear stress exerted by the material flow in the extrusion process. The greater strength hardening effect accomplished with orientation of cell along the extrusion direction and surface energy associated with the cell having an oval cross section are closely related to the higher compressive strength of plastic foam structure along the extrusion direction.
In FIG. 1, maximum compressive strength of polyethylene foam sheet is parallel to direction Y provided by shearing force of foam sheet extrusion and the cell orientation of cell group of a conventional extruded polyethylene foam sheet wherein the cells are elongated in parallel alignment with vector Y along which the foam sheet is extruded as shown in FIG. 2, a magnification of the foam sheet section in the circle of FIG. 1.
It is one object of the invention to provide a polyolefin foam sports board with improved compressive strength by the use of a stiffening element having high compressive strength in the Y direction.
It is yet another object of the invention to provide a low density foam board having high flexural strength in the Y direction by the use of a stiffening element.
It is yet another object of the invention to provide an improved foam board having a polyolefin foam core coupled with light weight stiffening element composed of substantially polymer foam material, wherein the polyolefin foam core and the stiffening element can be integrally heat bonded.
It is yet another object of the invention to provide an improved foam board having a polyethylene foam core coupled with stiffening element adapted to increase the stiffness of a selected portion of the board.
It is yet another object of the invention to provide an improved foam core that has the structure stiffness advantages of a polystyrene foam core in the main body and the desirable resilient and shock absorption properties of polyethylene foam in the top and front portion of the board.
It is yet another object of the invention to provide an improved bodyboard with stiffening element adapted to provide the resistance to flex along a transverse axis and yet allow adequate flexibility along a longitudinal axis of the foam core in the forward quarter of the board.